Scourge (xenophagia transmorphus)

Probably one of the most terrifying aliens known to man, or any other species. No one has ever seen what a purebred, adult Scourge looks like. These aliens attack other species with vicious claws. Victims of such attacks with either be killed outright, or become the host to a larval Scourge. The larva will burrow its way through the host, feeding on what it can until it reaches the brain. Once there the larva goes into a dormant stage while it integrates the hosts DNA with its own. Finally, once matured, the larva will awaken and burst out of the host's head, leaving the larva a crossbreed of Scourge and whatever the host species was. There have been reports of another form of Scourge that is not dependent on other life forms in the same parasitic way as most Scourge seem to be. The accounts on this variation are rare, and very sketchy at best.